For example, some authors reported that FGF21 and GDF15 were correlated with PMD severity and could be used as reliable indexes of disease progression, but Koene et al. showed that the two biomarkers were not correlated with disease severity in a large cohort of adult m.3243A>G mutation carriers (20, 21). Here, FGF21 is linked to Pelizeaus-Merzbacher spectrum disorder.